This three-wheeled car runs on gas fumes. An entry for the Progressive Automotive X Prize, a contest for the most fuel-efficient car, it was on display at the New York International Auto Show March 18.

Drivers often joke their car "is running on fumes," when the tank gets low. Well, how about an engine that actually gets its energy from gasoline fumes?

Or, tired of looking for parking space? Well, someone has dreamed up an all-electric car so thin it can compete with motorcycles for the gaps between SUVs.

And no need to fill up on expensive gasoline anymore – one would-be Henry Ford wants to build an engine that runs on compressed air, the stuff that fills your tires. Naturally, it's called the Air Car.

All of these ideas – some with actual tires on the ground – are entered for the Progressive Automotive X Prize, an international competition that will award $10 million to the first team that can build and bring to market a car that gets the equivalent of 100 miles per gallon. So far, no major auto company has said it'll compete, although some have said they are curious and might kick a few tires when no one is looking.

There will be a contest, scheduled to begin September next year, in which these ambitious prototypes will be driven about like regular cars. And since Americans like to race – or at least go to NASCAR events – these decidedly non-Detroit vehicles will hold a race of their own.

Last Thursday, the X Prize Foundation rolled out some of the contestants at the New York International Auto Show, an event where "concept cars" usually just mean futuristic styling, or technology that won't be ready for the public to use until colonies are established on the moon.

"We need a car that is not just a concept but can be made in mass quantities at a reasonable cost for the average American," says Jack Hidary, chairman of the Coalition Advocating for Smart Transportation and a donor to the X Prize's new effort. "Unfortunately, Detroit has not stepped up to the plate, they have fought CAFE [Corporate Average Fuel Economy] standards every step of the way."

"In 2001, we put a European Lupo3L hypereconomy car through the now-archaic EPA testing and got 80 miles per gallon in the city and 100 on the highway," says Keith Price, the public relations manager. "So in terms of the X Prize, we wish them well but from our perspective, we've been there, done that."

The Lupo is no longer in production.

Another manufacturer, Honda, is quite pleased with the ecological virtues of its hydrogen fuel-cell car, which gets 62 miles per gallon. As he shows off the vehicle, company spokesman Todd Mittleman reveals that the seats are made of corn-based bio-fabric. "So, I guess if you are stuck in your car for a couple of days, you can eat the seats, although I wouldn't recommend it," he says.

What about the X Prize? Mr. Mittleman says he would like to be present when the winner is announced even though Honda so far hasn't indicated it would enter the contest. "The constraints and criteria are pretty tough," he says.

Their gas fume car – conceived and built in spare hours outside full-time jobs – has so far gotten up to 92 miles per gallon. Now, Mr. Pratt, who owns restaurants, and his cohorts (or is that carhorts?) are moving toward a combination of vapor technology and electric hybrid to get to the "magic" 100 miles per gallon.

Have they had any bites from automakers?

"No, we're very much under the radar," says Pratt, whose group has been working on their unusual-looking three-wheeled vehicle for two years. "We don't have any illusions, we want to be a small niche automaker, starting with 150 to 200 cars a year and then maybe working up to 5,000 or 10,000 cars a year while developing technology and licensing it out to others."

The 60 teams from nine countries that have entered the competition include a team from an inner city high school in Philadelphia, a group from Cornell University, and a company called "Psycho-Active" in Moore, S.C.

"The idea behind the name is about thinking, pushing the paradigm," says John Robitaille, the leader of the Moore team, which is working on an advanced rotary engine design.

But will people want to drive a Psycho-Active? "It doesn't roll off the tongue," admits Mr. Robitaille, who also has a day job in the telecom business.

The only car company that has signed on is Tesla Motors, which makes a 100 percent electric sports car that the company claims gets the equivalent of 135 miles per gallon. The base price for the 2008 model is $98,000.

The price alone may make it difficult to win not only the X Prize but also the hearts and minds of American drivers.

As he sits in a VW at the auto show, Christopher Hopson, an auto analyst at Global Insight, an economic forecasting firm in Lexington, Mass. says the X Prize could be good for the planet. But, he warns contenders to make their cars affordable. "If it's too expensive, that's a barrier," he says.